The recognition of antigenic epitopes presented by molecules of the Major Histocompatibility Complex (MHC) plays a central role in the establishment, maintenance and execution of mammalian immune responses. T cell surveillance and recognition of peptide antigens presented by cell surface MHC molecules expressed by somatic cells and antigen presenting leukocytes functions to control invasion by infectious organisms such as viruses, bacteria, and parasites. In addition it has now been demonstrated that antigen-specific cytotoxic T lymphocytes (CTLs) can recognize certain cancer cell antigens and attack cells expressing these antigens. This T cell activity provides a basis for developing novel strategies for anti-cancer vaccines. Furthermore, inappropriate T cell activation plays a central role in certain debilitating autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and asthma. Thus presentation and recognition of antigenic epitopes presented by MHC molecules play a central role in mediating immune responses in multiple pathological conditions.
Tumor specific T cells, derived from cancer patients, will bind and lyse tumor cells. This specificity is based on their ability to recognize short amino acid sequences (epitopes) presented on the surface of the tumor cells by MHC class I and, in some cell types, class II molecules. These epitopes are derived from the proteolytic degradation of intracellular proteins called tumor antigens encoded by genes that are either uniquely or aberrantly expressed in tumor or cancer cells.
The availability of specific anti-tumor T cells has enabled the identification of tumor antigens and subsequently the generation of cancer vaccines designed to provoke an anti-tumor immune response. Anti-tumor T cells are localized within cancer patients, including in the blood (where they can be found in the peripheral blood mononuclear cell fraction), in primary and secondary lymphoid tissue, e.g., the spleen, in ascites fluid in ovarian cancer patients (tumor associated lymphocytes or TALs) or within the tumor itself (tumor infiltrating lymphocytes or TILs). Of these, TILs have been the most useful in the identification of tumor antigens and tumor antigen-derived peptides recognized by T cells.
Conventional methods to generate TILs involve mincing tumor biopsy tissue and culturing the cell suspension in vitro in the presence of the T cell growth factor interleukin 2 (IL-2). Over a period of several days, the combination of the tumor cells and IL-2 can stimulate the proliferation of tumor specific T cells at the expense of tumor cells. In this way, the T cell population is expanded. The T cells derived from the first expansion are subsequently mixed with either mitomycin C-treated or irradiated tumor cells and cultured in vitro with IL-2 to promote further proliferation and enrichment of tumor reactive T cells. After several rounds of in vitro expansion, a potent anti-tumor T cell population can be recovered and used to identify tumor antigens via conventional but tedious expression cloning methodology. Kawakami Y. et al. (1994) Proc. Natl. Acad. Sci. USA 91(9):3515–3519.
This currently employed methodology used to generate tumor specific T cells in vitro is unreliable and the antigens identified by this method do not necessarily induce an anti-tumor immune response. Numerous experiments demonstrate that the encounter of antigens by mature T cells often results in the induction of tolerance because of ignorance, anergy or physical deletion. Pardoll (1998) Nature Med. 4(5):525–531.
The ability of a particular peptide to function as a T cell epitope requires that it bind effectively to the antigen presenting domain of an MHC molecule and also that it display an appropriate set of amino acids that can be specifically recognized by a T cell receptor molecule. While it is possible to identify natural T cell epitopes derived from antigenic polypeptides, these peptide epitopes do not necessarily represent antigens that are optimized for inducing a particular immune response. In fact, it has been shown that it is possible to improve the effectiveness of natural epitopes by introducing single amino or multiple acids substitutions that alter their sequence (Valmori et al. (2000) J. Immunol 164(2):1125–1131). Thus, delivery of carefully optimized synthetic peptide epitopes has the potential to provide an improved method to induce a useful immune response.
The introduction into an animal of an antigen has been widely used for the purposes of modulating the immune response, or lack thereof, to the antigen for a variety of purposes. These include vaccination against pathogens, induction of an immune response to a cancerous cell, reduction of an allergic response, reduction of an immune response to a self antigen that occurs as a result of an autoimmune disorder, reduction of allograft rejection, and induction of an immune response to a self antigen for the purpose of contraception.
In the treatment of cancer, a variety of immunotherapeutic approaches have been taken to generate populations of cytotoxic T lymphocytes which specifically recognize and lyse tumor cells. Many of these approaches depend in part on identifying and characterizing tumor-specific antigens.
More recently, certain pathogen- and tumor-related proteins have been immunologically mimicked with synthetic peptides whose amino acid sequence corresponds to that of an antigenic determinant domain of the pathogen- or tumor-related protein. Despite these advances, peptide immunogens based on native sequences generally perform less than optimally with respect to inducing an immune response. Thus, a need exists for modified synthetic antigenic peptide epitopes with enhanced immunomodulatory properties. This invention satisfies this need and provides related advantages as well.